Arithmetic units for digital computers



Aug. 20, 1957 E. c. NELSON 2,

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ELDRED C. NELSON Aug. 20, 1957 E. C. NELSON ARITHMETIC UNITS FOR DIGITAL COMPUTERS Onginal Filed Oct. 10. 1950 13 SheeIs-Sheet 9 1957 E. c. NELSON 2,803,401

ARITHMETIC UNITS FDR DIGITAL COMPUTERS Onginal Filed 001:. 10, 1950 v 13 Sheets-Sheet 11 DELAY cmcun' 1- CLOCK 43m 2300 PULSE lolb 7 GENERATOR we-r FuLL-AbDER E 56 OR 2351 1w; FULL-SUBTRACTER (2350 2353 i D 9'5 OR "AND" FULtADDER-SUBTRACTER GATE g? i i ,2560 2372 FLo I II It 1 AND c ac 1 GATE i on BINARY TIME INTERVAL NSTORTED C smmu. 1

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ARITHMETIC UNITS FOR DIGITAL COMPUTERS Onginal Filed Oct. 10. 1950 13 Sheets-Sheet 12 ONE BINARY TIME #NT'ERVAL C+=A-B+A-D+B-D g 6 K- E+ -B +15 e,

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NELSON %,M 7744 EG-ZE United States Patent ARITHMETIC UNITS FOR DIGITAL COMPUTERS Eldred C. Nelson, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware- Continuation of abandoned application Serial No. 189,318, October 10, 1950. This application August 31, 1956, Serial No. 607,494

29 Claims. (Cl. 235-61) This invention relates generally to electronic computers and particularly to electronic apparatus for adding or subtracting quantities represented by voltage pulses or levels. This application is a continuation of application Serial No. 189,318 entitled, Arithmetic Units for Digital Computers, filed October 10, 1950 by Eldred C. Nelson, now abandoned.

The circuits of this invention perform computations with signals having distinct values representative of numerical digits, in contrast to that class of computing devices which use continuously varying signals as the variables of computation, i. e., the disclosed computer elements are of the digital rather than the analog type. All computations are performed in the binary system of numbers which employs only two digits, 0 and i.

As will be described in more detail later, the disclosed adding and subtracting circuits are composed of various combinations of the so-called and and or gate circuits. The physical properties of these and and or gates are described and rules for combining these two types of gates to produce any desired output signal respouse are given in terms of the basic response patterns of these two types of gates. It is shown that there are a number of possible combinations of an and or gates which will produce the same output signal response. Rules based on the physical properties of and and or" gates are given which permit the derivation of all possible combinations of these gates capable of producing a given output signal. The simplest circuits for producing the output signal responses used in addi tion and subtraction are disclosed in detail and it is then demonstrated how these circuits may be combined or enlarger! to obtain complete addition or subtraction of binary numbers.

The circuits disclosed have a physical structure consisting of resistors and diodes. The diodes may be of either the vacuum tube type or crystal type, butthe crystal diodes are preferable because of their smaller size, greater reliability, lower cost, and shorter time delay as compared with the vacuum tube type. By reason of the insignficant time delays introduced by the crystal diodes, quite complex computer networks may be designed for operation at very high speeds.

Simple networks of and and or gates are disclosed which produce output signals corresponding to the sum of two binary digits. Other networks produce output signals corresponding to the difiereuce of two binary digits. Still other networks produce output signals corresponding to either the sum or difierence of two binary digits, depending on the position of a switching device. These relatively simple networks may be combined to form more complex networks which will add or subtract two binary numbers, each number consisting of a series of binary The more complex adder circuits automatically add the carry digit from the addition of one pair of digits (one digit from each binary number} to the sum of the next pair of binary digits. The more complex subtracter circuits Patented Aug. 20, 1957 automatically include the carry digits of subtraction. Two general types of these more complex adders and subtracters are disclosed. The first type is known as a series circuit. In this type two time-varying signals representing the two binary numbers to be added or subtracted are fed into the adder or subtracter and the digits of the sum or difference appear in time sequence as a time-varying output signal. The second type is known as a parallel circuit, in this second type each binary digit of the two numbers to be added or subtracted is represented by a diuerent input signal and all of these input signals are fed into the parallel device simultaneously. The digits of the sum or difierence of the two binary numbers appear simultaneously at the output. The series type oi circuit requires fewer circuit components than the parallel type, but the series type is proportionately slower than the parallel type in performing additions or subtractions. In the series type of adder or subtracter, in order to combine the carry digit signal at the proper time, it is necessary to use a memory circuit or delay line to store this carry digit signal for a predetermined length of time before combining it with the signals representing the next pair of digits to be added or subtracted.

It is, therefore, an object of this invention to provide networks composed of novel combinations of and and or circuits which are capable of adding or subtracting binary numbers in a rapid, accurate, and simple manner.

Another object of this invention is to provide networks composed of an and or circuits which produce output signal responses which correspond to the result and carry digits of a binary addition or subtraction of two binary digits which are represented by two input signals.

Another object of this invention is to provide networks composed of and and or circuits which give an output signal response which corresponds to the result digit of a binary addition or subtraction and another output signal response which corresponds to the carry digit of addition when a switching device is in one position and to the carry digit of subtraction when said switching device is in its other position.

Another object of this invention is to provide networks composed of *and" and or circuits, plus a delay circuit, which produce a time-varying output signal response corresponding to the digits of the binary sum or difference of two binary numbers which are represented by two time-varying input signals.

Another object of this invention is to provide networks composed of and and or circuits which produce a series of output signals corresponding to the digits of the binary sum or difference of two binary numbers, the digits of said binary numbers being represented by a series of input signals.

Still another object of this invention is to provide networks composed of diodes and resistors, said network being adapted to produce output signal responses which corresponding to the sum or difference of two binary numbers represented by the input signals to said network, these signals, normally having only two voltage levels, corresponding to the ed and 011" positions of the input circuits.

The invention and its objects will be better understood from the following description considered in connection with the accompanying drawings, in which several embodiments of the invention are illustrated by way of examples. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

Fig. l is a composite diagram of signals which may be used for operating the circuits disclosed in this application;

Fig. 2 is a block diagram of a fiip-flop circuit to produce signals suitable for the binary adders and sub tracters disclosed in this application;

Fig. 3 is a schematic diagram of an *and" circuit;

Fig. 4 is a schematic diagram of an or circuit;

Figs. 5, 6, 7 are schematic diagrams of circuits which produce identical output signals, but which use different circuit elements and have dilferent connections to the sources of input signals;

Fig. 8 is a schematic diagram of an N-terminal and circuit;

Fig. 9 is a schematic diagram of an N-terminal orl circuit;

Figs. 10 and 11 are schematic diagrams of difierent half-adder circuits;

Figs. 12 and 13 are schematic diagrams of different half-subtracter circuits;

Figs. 14 and 15 are schematic diagrams of two circuits, each of Which may function either as a half-adder or as a half-subtracter;

Fig. 16 is a schematic diagram of a half-adder which produces the complementary output signals;

Fig. 17 is a schematic diagram of a circuit which may function either as a half-adder or as a half-subtracter. and which produces the complementary output signals;

Fig. 18 is a block diagram of a complete series adder circuit;

Fig. 19 is a schematic diagram of a complete series adder circuit;

Fig. 20 is a schematic diagram of a circuit which may function as an adder or as a subtracter;

Fig. 21 is a schematic diagram of a circuit which may function either as an adder or a subtracter, and which produces the complementary output signals;

Fig. 22 is a block diagram of a complete parallel adder;

Fig. 23 is a block diagram of a series adder with a flip-flop type of delay circuit;

Figs. 24 and 25 show the waveforms of signals appearing at various points in a circuit of the type shown in Fig. 23;

Fig. 26 is a schematic diagram of a series adder-subtracter having modified carry signal networks and a flipilop type of delay circuit.

The circuits which are disclosed in this application can be explained most conveniently by taking up the following topics in the order indicated:

1. The signals used in the disclosed computer circuits;

2. The physical properties of and and or gates having two input terminals;

3. A method of representing the operation of and and or gates which have two input terminals symbolically;

4. And and or gates which have more than two input terminals and the symbolic representation of these circuits;

5. Half adding and subtracting circuits, the simplest forms of adding and subtracting circuits that can be made up of and and or gates;

6. Series adding and subtracting circuits;

'7. Parallel adding and subtracting circuits;

8. Delay circuits used in series adding and subtract ing circuits.

The remainder of the disclosure will be divided into eight sections having the titles indicated above.

THE SIGNALS USED IN THE DISCLOSED COM- PUTER CIRCUITS The computations of the computer elements of this invention are made in the binary system of numbers which employs only two digits, 0 and 1. A complete discussion of this system of numeration is given in Elementary Number Theory by Uspenski and Heaslet, published in 1939 by McGraw-Hill Book Company, Inc., New York and London. The use of this system makes it necessary to distinguish only two values of signal level throughout the computer, for instance E1 and E2. The binary digit 1 may be arbitrarily correlated with the higher of the voltages, E1, and the binary digit 0 may be arbitrarily correlated with the lower of the voltage values, E2 (or vice versa). In the identification of a signal by a computer element utilizing the binary system of numbers, only the nearness of its voltage to E1 or E2 need be distinguished. Considerable variation in the values of E1 or E2 may be permitted without altering the reliability of the discrimination; hence the use of precision components can be avoided and small changes in the circuit components will not seriously affect computer operation. If a decimal system of computation were used, for example, it would be necessary to distinguish 10 separate voltage levels, one level corresponding to each decimal digit. Hence the type of signal used by the computer elements of this invention is one that gives very reliable operation.

In the dynamic operation of the computer, numbers and control signals are represented as time sequences of binary digits. A certain basic time interval is selected and each such time interval in a given period of time is a space for a binary digita binary time interval. Thus a signal in the computer will consist of a time varying voltage which varies between E1 and E2. If it rises to the value of E1 during a binary time interval, the binary digit in that time interval is 1; if it remains at E2 during the entire binary time interval, the binary digit in that time interval is 0.

Two types of wave forms of the voltage signal may be used in computers. In one type, the binary digit 1 is represented by a voltage pulse; i. e., the voltage rises from E2 to E1 and falls back to E2 in a time smaller than the binary time interval. In the other type, the binary digit 1 is represented by a voltage state, the voltage rising to the value E1 and remaining there through out the binary time interval. In both types, the binary digit 0 is represented by the voltage being at the value E2 throughout the binary time interval.

The output-input signal relation for the diode gates is independent of the type of signals used; i. e., the input and output signals will have the same representation in terms of binary digits regardless of whether voltage pulses or voltage states are used.

The two types of signals, discussed above, are shown in Fig. l. Both types of signals may be used in one computer. In magnetic storage drums which serve as memory units in some digital computers, it has been common to use the equivalent of the voltage state signal, a magnetic flux state signal. Data is stored on a magnetic storage drum until ready for use and is then detected by means of a magnetic head. This magnetic head responds only to changes in magnetic flux density. Hence the signal shown in Fig. l as a voltage state, if stored on a magnetic drum as a flux state would cause an output signal to appear on a positive pulse at the beginning and a negative pulse at the end of the first magnetic flux state as shown in Fig. l. The output of the magnetic head, when scanning the next magnetic fiux state lasting for three binary time intervals as shown in Fig. 1, gives a positive pulse at the beginning and a negative pulse at the end of the flux state. Thus, these output signal pulses contain the same information as the magnetic flux state signal, however, they are not in proper form for use in a computer circuit. If the computer circuit (adder, subtracter, etc.) is designed to use the pulse type signal, as is generally true in the prior art, it is necessary to use the output of the storage drum to operate a gate circuit which opens or close whenever a pulse is received from the storage drum. A clock circuit must then be used to supply pulses through the open gate to the computer circuit. The invention described herein otters an improved method of utilizing the output of a storage device such as that described above, or any device with a similar output, by eliminating the necessity of this particular clock circuit. The omission of this clock circuit is made possible by the fact that the circuits of this invention will operate on either the pulse or voltage state signal types. Hence it is possible to use the output of the storage device to key a simple flip-flop circuit which then produces the desired voltage state signals for the computer directly. This arrangement is shown in Fig. 2 in block diagram form. However, it should be pointed out that the use of clock circuits in computer circuits employing the computer elements of this invention may still be desirable to time the recording of the signals on the drum.

The flip-flop circuit of Fig. 2 may be any one of many types of circuits, such as the Eccles-Jordan circuit. Basically, the flip-flop circuit is a device which has two stable states, corresponding to the voltage state signals described above. The flip-flop circuit may be designed to change from one state to another whenever a pulse is received at the input. The flip-flop of Fig. 2 responds to either positive or negative pulses. The particular type of flip-flop circuit employed is deemed immaterial to the disclosure of this invention, hence no separate circuit diagrams of flip-flop circuits have been disclosed.

There will be two outputs from the flip-flop circuit as shown, signal A at terminal a" which is a reproduction of the magnetic flux state signal on the drum, and signal A at terminal (2" This signal A has the value 1 when signal A has the value it has the value 0 when signal A has the value 1. The signal A is called the signal complementary to signal A. This notation will be followed throughout the disclosure, describing signals by letters, A, B, C, V, etc. and denoting their respective complementary signals A, B, C, V, etc. The two outputs of the flip-flop circuit may be applied directly to the input terminals of the computing circuits described herein which properly respond to the voltage state signals so generated. Thus an important feature of this invention is the simplification introduced by the fact that the signals used in the computer circuits may be generated directly by a flip-flop circuit, instead of by the use of clock and gating circuits.

THE PHYSICAL PROPERTIES OF AND AND OR GATES The basic circuits which form the building blocks of the adding and subtracting circuits of this invention are shown in Figs. 3 and 4. These circuits have found use in the prior art in the following applications: reshaping of deteriorated pulses produced by various elements of electronic computers, selecting or inhibiting a certain one or groups of pulses from a pulse train, combining or isolating the outputs of several sources with difierent output impedances, and as selector circuits for use in changing binary numbers into decimal numbers at the input or output of a computer. The present invention consists primarily of the combination of these elementary gate circuits to form adders and subtracters.

The circuit shown in Fig. 3 is known as an and circuit. Referring to Fig. 3, a battery B is shown having a positive and a negative terminal. The negative terminal is connected directly to a negative bus 302. The positive terminal is connected to a gate resistor, Re, which in turn is connected by a lead 313 to output terminal .r. Output resistor R5 is connected at one end to output terminal s and is connected at its other end to bus 302. The anode terminal of two crystal diodes, De, Db, are connected to conductor 313. The cathode terminals of the diodes are connected to the input terminals of the and" circuit, a, b, respectively. Terminals a, b, are the output terminals of switches 314, 315, respectively, which in turn include movable switch arms 310, 311, respectively, and fixed contact points 305, 303, and

6 304, 306, respectively. Fixed contact points 305, 306 of switches 314, 315, respectively, are tied together and are connected by a conductor 312 to the positive terminal of the battery B While fixed switch points 303, 304 of switches 314, 315, respectively, are connected directly to negative bus 302.

The gate resistor, R0, has a high resistance compared with the forward resistance of diodes Da, and Db. In practice, the diode forward resistances are of the order of several hundred ohms if the diodes are of the germanium crystal type. Resistor Rs is high compared with resistor R0, as are the back resistances of Des and Db- In practice the resistance of resistor R0 is usually in the range of 15,000 to 100,000 ohms and that of resistor R about 1 megohm.

The operation of the circuit is as follows: When both input terminals a, b are connected to the negative terminal of the battery, current flows from the positive terminal of the battery through resistor R0 and then divides between the diodes and resistor R5 with the major portion of the current flowing through diodes Da. and Db. Most of the voltage drop occurs across resistor R0 since its resistance is much greater than the diode forward resistance, hence the terminal "s is nearly at zero potential as indicated in Table I.

When terminal a has volts applied and b is connected to the negative terminal of the battery, the current flows through resistor R0 from the positive terminal of the battery and from terminal a through the back resistance of diode Dr; to the junction s, at which point the current divides between resistor Rs and the forward resistance of diode Db and flows to ground. Now the parallel combination of resistor R5 and the forward resistance of diode Db is essentially the resistance of diode Db since the resistance of resistor R5 is much greater than the resistance Db. Also both resistor R0 and the back resistance of diode Do are much greater than the forward resistance of diode Db, and hence most of the voltage drop will occur across resistor R0 and the back resistance of diode D8- So again terminal s is essentially at zero potential. The same result is obtained if the voltages on a and b are reversed since the circuit is symmetrical.

When a and b are both at 100 volts, current flows through the parallel combination of resistor R0 and the back resistances of both diodes to s and from s through resistor R5 to ground. The back resistances of the diodes are much greater than the resistance of resistor R0 so the parallel combination of these three resistances is essentially equal to the resistance of resistor R0. Now the resistance of resistor Rs is much greater than that of resistor R0 and, hence, most of the voltage drop will occur across resistor R5 and the voltage at s will be nearly the battery voltage, here assumed to be 100 volts.

The circuit disclosed in Fig. 3 is known in the computer art as an and" circuit because the output voltage will be high only if terminal a and terminal b are connected to high voltage. Therefore, the circuit in Fig. 3 may be defined as follows: If, and only if, the voltage at terminal a and the voltage at terminal 5 are high, will the output voltage be high. if either or both input voltages are low, then the output voltage will be low.

In Fig. 3 and the subsequent figures of this disclosure the input signals are shown as being supplied from a battery and one or more switches which are connectable to either terminal of the battery. This arrangement is merely shown for the purpose of explanation, and is not to be understood as being a preferred embodiment. In actual practice electronic *switches" in the form of flipflops or other circuits would be used to supply the input signals. Since the particular type of signal source is immaterial to the discussion of this invention, a form of signal source which is simple to understand has been illustrated throughout this disclosure.

Fig. 4 is a diagram of an or circuit. it differs from the circuit of Fig. 3 in that the battery and the diode connections are reversed, so that the current flows through the diodes and resistor R in the opposite direction. Referring to Fig. 4, the cathode terminals of diodes Ds and Db are now connected to lead 313 and the anode termi nals are connected to input terminals a, 1:. Lead 313 is connected to one end of resistor RC. and the other end of resistor R0 is connected to the negative terminal of the battery through a lead 400. The other connections are as in Fig. 3. The relationships between the values of the resistances arc the same as for the and circuit.

The operation of the or" circuit will be as indicated in Table 2.

Table 2 SignalA: i SignalB:

This table may be explained as follows:

if the voltages at a and b are both zero, it is clear that no current will ilow and s will also be at a zero potential. It the voltage at a is l00 volts and that at b is zero, current flows through the forward resistance of diode Dal dividing at the junction with lead 313 and flowing through resistors Rs, 1%.. and the buck resistance of diode Du to ground. Since the parallel combination of these three resistances is much greater than the forward resistance of diode Du, most of the voltage drop will occur across resistors R0 and R and diode Db, and the potential at 5 will be nearly the battery voltage of 100 volts. if the voltages on a and b are reversed, the result is the same, the circuit being symmetrical. if both a and b are at 100 volts, current flows through the forward resistances of both diodes and divides between resistors R0 and Rs in flowing to ground. Again. most of the voltage drop is across the parallel combination of resistors Rn and Rs and the voltage at s almost the same as that at a and b.

The or circuit may be defined in words as follows:

if either or both input voltages are high, then the output voltages will also be high. if, and only if, both input voltages are low, will the output voltage be low. he same circuit is also defined as follows: if either the voltage at terminal a is higher or the voltage at terminal b is high, then the output voltage will be high.

If we consider voltages near 0 volts to correspond to the binary digit 0, and voltages near +100 volts to correspond to the binary digit 1 in Tables 1 and 2, we may define the operation of the and circuit as in Table 3 and the or" circuit as in Table 4.

Table 4 A METHOD OF REPRESENTING THE OPERA- TION OF AND" AND OR GATES SYM- BOLICALLY The circuits of Figs. 10 through 22 all include and and or gates. In order to simplify the discussion of these more complex circuits, a symbolic notation for the operation of and anc or gates will be introduced. This method of describing the simple gate circuits will make it possible to use very simple algebraic expressions to describe completely the complex adding and subtracting circuits of Figs. 10 through 22, both as to circuit con nections and as to the form of output signal which is produced.

The operation of the elementary gates of Figs. 3 and 4 can be described symbolically, since each gate receives the input signals, A and B, and generates an output sig nal, S, which depends on the signals A and B. The output signal is a signal of the same type as the input signals.

Mathematically, the gates can be considered as performing a binary operation on the input signals which yields an output signal that is determined by the input signals, the correspondence between the input and output signals being given in Tables 1 and 2. The gate shown in Fig. 3 performs one binary operation, which may be denoted {A, B}, and the gate shown in Fig. 4 performs anothcr binary operation, denoted [A, B], on the signals A and B. In other terms, the output signal, S, of an and" gate may be considered to be a function of the two input signals A and B, this function being defined by Table 3 and the mathematical expression for the function being given by the symbols S={A, B}. Similarly the or gate output signal is a function of the two input signals which may be stated mathematically as S lA, B]. The use here of the word function is just as in ordinary mathe matics. The definition is: A function of a signal variable A is a rule that assigns to each value of A (0 or 1), a value F(A)" (which also is 0 or l); e. g, the complement, A, of a signal, A, can be considered as a function of A.

Table 5.

Table 5 A F1 F2 They may be written in equation form by inspection as:

Two of these functions, F1 and F4, are trivial. Circuit- 9 wise, they need not involve any connection to the source of the signal A. F1 can be produced by means of a wire connected to the source of low voltage; F1 can be produced by means of a wire connected to the source of high 10 F3, F4, and F5 also may be produced by and gates, the necessary signal inputs being, by inspection voltage. 5

Figs. 10 through 17 show adding and subtracting cir- FM B)={A E} cults which produce output signals which are functions of two input signal variables. The explanation of these FHA B)={A,B} circuits will be simplified by the discussion that follows.

Functions of two variables are defined in a manner simil lttnlfthro lar to that for a singliigvanable. A function of two Isignal 33h 2 p t gigngfsilizes. a It Fri 1335116515 (2)1 Silg n :11 oily i; 323 3;: g xzzg f ii'iifig gfi ifig g g 13i both input signals are 0; hence those output signals that teen possible functions of two variables; i. e., there are 0 for only one 95 the combmatmfls 9f binary sixteen possible ways of assigmng each pair of values 15 digits 1 5 3 0f the combinations) @5111! be P of A and B a value F(A, B) which 1s 0 or 1. These duced from or gates. F12, F11, F14, and F15 are funcfunctions are shownin Table 6. tions of this type. The connections of the or gates Table 6 A 13 F1 F: F1 F4 5 F0 F1 Fa Fa F10 F11 F1: F1: u F15 F10 I o 11 0 1 o a 0 1 1 1 0 o 0 1 1 1 0 1 0 t 0 0 1 0 0 1 o 0 1 1 o 1 1 0 1 1 1 1 o 0 0 ti 1 0 u 1 o 1 1 o 1 1 1 1 The function F5 may be recognized as that produced by to the signal inputs are defined by the expressions: the and gate, and the function F15 may be recognized as 0 B E that produced by the or gate. In terms of the notation F1204 )zEA, which is used above F13(AB)=[Z,B]

Fit-4A, B)=[A, B] 1 bl b h rm 0th bl F15(AB)=[AB] t is possi e to o tain one o e er possl e output signal functions by means of combinations of and and g g f f ggf ig gg gg g i ggg fig fi gf or gates propfa'rly i i It f the signal when the A, B signal pair requires a 0 output in complementary. Input i A and are Madame i 40 Table 6. For example, F13 requires a 0 signal when g g: gg f g i i i fig g gg gg-gg g 3 5%; A=l and 3:0. An of" gate which meets this require- 1 E d c n o ment must have both input signals cquai to 0 when Azl signs may ease and 8 0, hence signals A and B are used.

The circuits connections required pl'oduca the More complex circuits are required to produce the output Sign-a1 funquons aboye may e denvd as fonows' functions Pa and F9. First consider the circuit for Fs. g z g gi gg a B Signals This function has a 1 signal for the same A, B signal 11, an, 16 y 6 0 at that produces a 1 in F2. it also has a 1 signal corre- F1(A B)=o sponding to the l in F5. If an or gate is connected F B to receive as input signals F2 and F5 it will produce an L- output signal Fa, since a 1 signal at either input of an The circuit necessary to produce these output signals is, in each case, simply a wire connecting the source of the specified signal to. the output sig terminal. Thus F1 is produced by a wire to the source of low voltage, Fs by a wire to the source of signalK, etc.

The other signals require the use of and" and or gates. The and gate produces a 1 output signal only if both of the input signals are 1; otherwise it produces a 0 signal; hence those output signals that are 1 only for one of the combinations of two binary digits can be formed from and gates. Tov produce signal P: which is a signal of this type, an and" gate may be used to which the signals A and B are supplied. A and 1 3 are 1 when A and B are d, hence a 1 output signal will be produced by this circuit when A and B are 0, as required. For no other A.B signal will A and B be 1, hence the output of this and gate will be 9 for the other A,B signals. F2 may then be written as mons qij} or gate produces a 1 output signal. Thus to produce Fa all that is required is a network composed of two and gates to form the signals F2 and F5 and an "or gate to combine the outputs of the two and gates. This network may be expressed in terms of the functions F: and F5 as follows indicating an *or" gate with inputs F2 and F5. The complete circuit for the network can be expressed in terms of the input signals as follows indicating two and" gates having input signals AB and A,B, respectively, feeding into an or" gate.

The circuit for F9 may be analyzed similarly. Fa has 1 signals corresponding to the l signals of F3 and F4. Hence an or" gate supplied with P3 and F4 as input signals will produce P9. In equation form and writing out F3 and F4 which indicates two and gates having input signals A,B and A,B, respectively, feeding into an "or" gate. Thus all possible functions of two variables may be produced either by direct connection to the sources of input signals or by simple combinations of and and or gates. The expressions for the output signal functions show the types of circuits required, the connections of the circuit to the sources of input signals, and the interconnections of the circuits.

In general, there are a number of circuits which will generate the same output signal. Each of these circuits corresponds to a different expression for the same output signal function. In order to obtain the class of circuits which will produce a particular output signal, certain properties of and and or gates must be investi gated. To facilitate this investigation a form of algebra known to mathematicians as Boolean algebra will be used. (See Birkhoff and MacLane, A Survey of Modern Algebra, pp. 331432, published by The Macmillan Co. of New York in 1949.) The rules of this system of algebra correspond exactly to the operations performed by the and and or gates. Only two operations are defined in this algebra. It is customary, in algebra, when a system has two operations to call them addition and multiplication. From Table 1 it can be seen that the operation of the and gate has the properties of ordinary multiplication. It will be called logical multiplication and denoted symbolically by a dot; i. e., the operation that has been denoted in previous paragraphs by {A,B} will be denoted by AB in the remainder of this disclosure.

The rules for logical multiplication of signals are given in Table 7.

Table 7 The operation of the or gate, as shown in Table 2, has some of the properties of ordinary addition. it will be called logical addition and will be denoted symbolically by a sign; i. e., the binary operation denoted in the preceding paragraphs by [A,B] will be denoted in the remainder of this disclosure by A-l-B. The rules for logical addition of signals are given in Table 8.

The last rule, 1+1=1, differs from the ordinary rule for addition.

in ordinary algebra, the operations inverse to addition and multiplication, viz., subtraction and division, are defined. In this algebra, the inverse operations do not exist, hence the cancellation laws for addition and multiplication do not hold; i. e., from A-l-B:A+C, it cannot be inferred that B=C and from A.B=A.C, it cannot be inferred that B=C.

The rules for logical multiplication and addition of computer signals may be used to derive the identities given in Equations 1 through 7 below.

Equation 5 is known as the distributive law of Boolean algebra and Equations 6 and 7 are known as the associative laws.

Since these identities are derived from the rules of operation of the and and or gates, it is assured that they validly define equivalent circuits, i. e., circuits which produce the same output signal. For example, Equation 1 means that the output signal which is 1 at all times may be obtained either by a wire from the source of high voltage or from the output of an or gate having input signals A and A applied. These identities state seven important properties of and and or gates. The mathemotical steps involved in their derivation are quite simplc. For example, Equation 1 can be checked as follows. When A is 1, A is O and their logical sum is 1. When A is l), A is l and the sum is l.

The physical meaning of identities (2) through (7) is as follows: Equation 2 means that if the signals A and A are applied to the input terminals of an and gate, the output signal will be 0. This equation and the subsequent equations may be checked by a process of reasoning similar to that above.

From Equations 1 and 2, it is clear that there are two ways of generating the signal functions F1 and F16 in Table 6. Instead of using a wire from the source of low voltage to produce F1, an and gate to which the signals A and A are applied could be used. Similarly an or" gate having signals A and A applied could be used in place of a wire to the source of high voltage to produce F16.

Equations 3 and 4 mean that if the signal A is applied to both input terminals of an or gate and the output signal will be A, and likewise if the circuit is an and gate. Equation 5 means that the output signal of an and gate receiving as input signals A and (B-l-C) is the same as the output signal of an or" gate receiving as input signals A.B and AC.

Equation 6 means that the output signal of an and" gate receiving two input signals, A and B.C, is the same as the output signal of an and gate having three input terminals and receiving three input signals, A, B, and C. And and or gates with more than two input terminals will be discussed more fully in connection with Figs. 8 and 9.

Equation 7 means that the output signal of an "or" gate receiving two input signals, A and B+C, is the same as the output signal of an "or gate having three input terminals and receiving three input signals, A, B, and C.

These identities thus represent equivalencies between certain and and "or gate circuit configurations on the basis of the output signal which is produced. These equivalencies were determined solely by use of the rules of logi cal addition and multiplication which describe the operation of and and or gates. The notation of Boolean algebra is used because it will be convenient in describing the more complex circuits.

The functions of Table 6 can now be written in terms of Boolean algebra notation, using a dot to indicate an and" circuit and a plus sign to indicate an or circuit, as follows i3 F1901, B)=B Fahd, B)=A The derivation of equivalent circuits to supply these signals will now be described in detail.

It has been noted that these functions of Table 6 can be written in several difierent ways; e. g., Fa, which has two ones and two zeros can be written as the sum F2 and F3, each of which has one one and three zeros.

Here the distributive property of Boolean algebra given in Equation 5, can be used.

9 Fs=1 5+3 Then Equation 1 may be used to simplify Fe. is F6=Z 1)=Z Thus, the same function, the same assignment of a functional value to the values of the arguments (in this case A and B), may have difierent algebraic forms, but these various algebraic forms can be transformed into each other by means of algebraic operations. Although these various algebraic forms represent the same function, i. e., the same set of signals, they correspond to diiferent circuits. The form for Fe in circuit form is just a wire connecting the source of signal A to the output signal terminal. To draw the circuits which correspond to Equation 7 and 8, the following rules may be followed: a plus sign between successive quantities of a series of signal quantities indicates an or circuit having its input terminals connected to the different points in the computer network at which the signal quantities appear; a dot between successive quantities of a series of signal quantities indicates an "and circuit having its input terminals connected to the different points in the computer network at which the signal quantities appear. Given an equation for the output signal of a circuit, it is possible to construct the circuit in every detail by use of the above rules.

The form (8), then has two and gates and one or gate; and the form (9) has one or gate and one and gate. The circuits corresponding to Equations 8, 9, and 10, respectively, are shown in Figs. 6, 7, and 5, respectively. Referring to Fig. 5, a battery B is disclosed having positive and negative terminals. The negative terminal is connected to negative bus 302 and the positive terminal is connected to fixed contact point 501 of switch 504. Negative bus 302 is connected to fixed contact point 502 of switch 504 and to one end of resistor Rs. The other end of resistor R5 is connected to output terminal s which in turn is connected by a wire 503 to movable switch arm 500 of switch 504. This wire 503 constitutes the circuit required by Equation 10. The signal A appears on movable switch arm 500, and the output signal Fa=A appears at terminal s.

Fig. 6 shows a battery B whose negative terminal is tied to negative bus 302 and whose positive terminal is connected to and circuit 1 composed of a resistor R61, diodes D611, Ds12, and a lead 600, and to and" circuit 2 which is parallel to and circuit 1 and is composed of a resistor R62, diodes D621, D622, and a lead 601. The leads 600, 601 of the and circuits 1 and 2 are connected to the anode terminals of diodes D631, D232, respectively, whose cathode terminals are connected together by a conductor 603 to form or circuit 3. The output of or" circuit 3 appears on conductor 603 and the output terminal s' which has a conductive path to ground bus 302 through the parallel resistors R63, R1. The cathodes of diodes D611, D621 are connected to movable switch arm 613 of switch 618. The movable switch arm 613 of switch 618 contacts either fixed contact point 612 which is connected to the positive terminal of the battery or fixed contact point 611 which is connected to negative bus 302. A double-pole double-throw switch 610 has one movable switch arm 605 connected to the cathode terminal of diode D612, and another movable switch arm 604 connected to the cathode terminal of diode D622. Arm 605 of switch 610 is adapted to contact either fixed contact point 609 which is connected directly to negative bus 302, or fixed contact point 608 which is connected to the positive terminal of the battery. Likewise, arm 604 of switch 610 is adapted to contact either fixed contact point 607 which is connected to fixed contact point 608 or fixed contact point 606 which is connected directly to negative bus 302.

The operation of this circuit is as follows: And circuit 1 forms the logical" signal product 2.1:; "and" circuit 2 forms the logical product EB; and or circuit 3 receives as inputs the output signals of and circuits 1 and 2 and forms the logical signal sum Z ill-1B, which is Fe. The Fa signal appears as the output of or circuit 3 at terminal s.

Fig. 7 shows a battery B having a positive terminal and a negative terminal, the negative terminal of which is tied to negative bus 302. The positive terminal of battery 3 is connected to and circuit 5 comprising a resistor R21, diodes D111, D712, and a lead 714, with lead 714 being connected to output terminal s. s is connected to resistor Rs which is, in turn, connected to ground bus 302. Or circuit 4, including a resistor R22, diodes D721, D122, and a lead 715, is connected at its resistor R22 end by a conductor 711 to negative bus 302, and is connected through lead 715 to the cathode terminal of diode D711. The anode terminals of diodes D121, D222 of or" circuit 4 are tied to the two movable switch arms 705, 702, respectively, of a double-pole double-throw switch 708. Movable switch arm 705 is adapted to contact either fixed contact point 707 which is connected to a conductor 710 which is in turn connected to the positive terminal of the battery, or fixed contact point 706 which is connected directly to negative bus 302. Movable arm 702 is adapted to contact either fixed contact point 704 which is connected directly to negative bus 302, or fixed contact point 703 which is connected to conductor 710. The cathode terminal of diode D212 is tied directly to movable switch arm 718 of a switch 716 and is thereby adapted to be connected to either fixed contact point 720 which is connected to conductor 710, or fixed contact point 719 which is connected directly to negative bus 302.

The operation of this circuit is as follows: Or cir cuit 4 forms the logical sum 8+1? And circuit 5 forms the logical product of K and the output of or circuit 4 to give Z.(B+), which is Fa, at output terminal s.

The same signal, Fe, can thus be generated by different circuits, the complexity of which varies considerably. By representing the output signal in algebraic form, the number and type of gates required to produce the specified signal is shown in convenient form. The derivation of other circuits which will produce the same output signal is made a matter of simple algebraic manipulation, once the equivalence between the configurations of and" and or gates defined by Equations 1 through 7 is known.

Each of the various algebraic forms of a given function corresponds to a different circuit, but all of these circuits produce the same output signal response to a given set of input signals. It is convenient to express the entire 

